A backlight module includes a backplane, a mold frame, a light guide plate, and an optic film. The optic film includes a first layer and a second layer stacked on the first layer and including a constraint portion including an extension section, a connection section, and a support section parallel with and opposite to the extension section. The connection section connects between the support section and the extension section. The extension section extends from an end of the second layer. The optic film is stacked on the light guide plate with the first layer engaging the light guide plate. The mold frame surrounds the light guide plate and the first layer and is received in the backplane. The connection section is located outside opposite sides of the backplane. The mold frame and the opposite sides of the backplane are received and retained between the support section and the extension section.